Tuned cavity device

ABSTRACT

1. A magnetically operable vibrating reed assembly comprising a support, an elongated reed having one end thereof secured to said support with the other end thereof remaining free to vibrate, said elongated reed having a plurality of longitudinally extending slots therein adjacent the free end of said reed to break up any radio frequency currents induced in said reed, coil means secured to said reed, means to supply an electrical current through said coil means, and means for the establishment of a magnetic field perpendicular to the longitudinal axis of said coil whereby said reed is caused to vibrate.

United States atent 1191 Burwell et al.

[ TUNED CAVITY DEVICE [75] Inventors: Bruce K. Burwell, Southport; Kenneth Powell, Horseheads, both of NY.

[73] Assignee: Westinghouse Electric Corporation,

Pittsburgh, Pa.

22 Filed: Feb.4,1963

211 Appl. No.: 255,759

52] U.S.Cl. ..315/39.59,315/39.61,331/5, ,332/5 51 rm.c1 ..H01j 25/50 [58] FieldoiSearch ..332/5;331/5; 315/3959, 39.61; 340/156, 8; 84/].06, 1.04, 1.13

- [56] References Cited UNITED STATES PATENTS -2,566,47s 9/1951 Becker. ..315/39.59 2,814,761 11/1957 Derbyetal ..315/3'9.59

[ Apr. 1Q, 1973 2,411,984 12/1946 Brown ..315/39.59 2,589,885 3/1952 Sonkin ..315/39.59 3,087,124 4/1963 McLeod ..331/5 2,750,565 6/1956 Mercer et a]. ..332/5 Primary ExaminerCar1 D. Quarforth Assistant Examiner-E. E. Lehmann Attorney--F. H. Henson, C. F. Renz and A. E. Renner EXEMPLARY CLAIM l. A magnetically operable vibrating reed assembly comprising a support, an elongated reed having one end thereof secured to said support with the other end thereof remaining free to vibrate, said elongated reed having a plurality of longitudinally extending slots therein adjacent the free end of said reed to break up any radio frequency currents induced in said reed, coil means secured to said reed, means to supply an electrical current through said coil means, and means for the establishment of a magnetic field perpendicular to the longitudinal axis of said coil whereby said reed is caused to vibrate;

9 Claims, 5 Drawing Figures PATENTEDF-PRIOIW 3,727,099

SHEETIUFZ @Qas a 64 WITNESSES 47 B INVENTORS d ruce K. urwell on K. Deon Powell ATTORNEY TUNED CAVITY DEVICE The present invention relates to resonant cavities and, more particularly, to resonant cavities employing means to vary the resonant frequency thereof.

The need has arisen, in several recent instances, for a resonant cavity or cavities which may be tuned in a periodic or stepped fashion to provide frequency modulation at command. While present requirements for such a step tuned cavity have largely been in the field of devices of the magnetron type, and while the ensuing discussion will be so directed, it is to be expressly understood that the present invention relating to the variation of the resonant frequency of a cavity has application not only in magnetron type tubes but also in any type of resonant cavity device in which it is desired to provide such tuning or frequency modulation. In the case of magnetrons, the need for a step tuned device has risen in radar systems where, for example, step tuning of a fixed change in frequency from pulse-to-pulse is desirable in order to give better definition of the target. Another use of such step tuning in the radar-magnetron field is to provide varying amounts of frequency modulation as the tube is tuned through the band in order to confuse the enemy discourage jamming.

It is, therefore, an object of this invention to provide improved means for varying the resonant frequency of a cavity.

Another object is to provide an improved means for varying the resonant frequency of a cavity in a periodic manner.

A further object is to provide an improved vibrating reed and assembly.

Another object is to provide an improved electron discharge device. I

A further object is to provide improved means for tuning resonant cavities.

Another object is to provide an improved electron discharge device of the magnetron type, the output frequency of which may be varied in a periodic manner. v

In brief, the present invention provides a vibrating reed which may be combined with a cavity device to vary the resonant frequency thereof. The reed is provided with at least one cavity member which is operable by the reed. Through proper means the physical vibration of the reed is effected whereby the reed serves to vary the degree of extension of the cavity tun ing member into the cavity. The placement of a tuning member into the cavity, of course, varies the frequency at which the cavity is resonant and, if desired, by causing the reed to vary at a prescribed frequency, the output of the resonant cavity is varied by a change in frequency in accordance with the vibration of the reed.

Further objects and advantages of the invention will become apparent as the following description proceeds and features of novelty which characterize the invention will be pointed out in particularity in the claims annexed to and forming a part of this specification.

For a better understanding of the invention, reference may be had to the accompanying drawings in which:

FIG. 1 is a topplan view, partially in section, of a magnetron type device embodying the present invention, the view being taken along line I-I of FIG. 2;

FIG. 2 is a side elevational view, partially in section taken along line IIII of FIG. 1; i

and to FIG. 3 is a top plan view of the reed utilized in the present invention;

FIG. 4 is a side elevational view of the reed of FIG. 3; and

FIG. 5 is a side elevational view, partially in section illustrating a modified embodiment of a portion of the device of FIGS. 1 and 2.

With reference now to FIGS. 1 and 2 there is shown a microwave device of the magnetron type incorporating the present invention. The device is inclusive of an anode indicated generally at 10. The anode 10 is comprised of three main portions including an outer cylindrical portion 12 and an inner washer or centrally apertured disk member 14 having an inner aperture 16. These two portions 12 and 14 are made of a suitable material, for example, copper and may be formed from a single block of material or may be separately made and then secured together by suitable means, for example, brazing. Circularly disposed about the inner aperture 16 are a plurality of vanes 18, the third portion of the anode 10. The region defined by the walls of two adjacent vanes 18 and that portion of the washer 14 disposed between two such adjacent vanes 18 define a resonant cavity 20. It is thus seen that the device has a plurality of resonant cavities 20 disposed about the inner aperture 16.

Centrally disposed within the aperture 16 is a cathode 22 for the emission of electrons into the device. The cathode 22 may be comprised of a tubular sleeve member 24 of suitable material such as nickel having disposed on the outer surface thereof a suitable electron emissive coating 26 for example; barium strontium oxide. Disposed within the sleeve 24 is a heater 28 which serves to raise the temperature of the coating 26 to emission temperature.

As illustrated in FIG. 1, there may be provided one or more tuning assemblies indicated generally at 30 to provide that the fundamental frequency of the magnetron may be tuned within a prescribed bandwidth. In the illustrated embodiment, four equally spaced tuning elements are shown. The assemblies 30 are shown comprised of an actuating plunger 31 to which is secured a probe member 32. The probe member 32 is disposed within a channel 34 located within the member 14. Each of the elements 30 may be connected to a single drive system to provide that by a single actuating means, not shown, the probes 32 are each positioned an equal distance towards the center of the device.

As viewed in FIG. 1, suitable output means 40, utilizing a wave guide 42 extending through the members 12 and 14 into one of the cavities 20, is provided to remove energy from the anode of the device.

As seen in FIG. 2, suitable means such as end plates 44 may be provided and vacuum sealed to the member 12 in order that the region including the cathode 22 and the cavities 20 may be maintained in a vacuum. Additionally, suitable magnet means 46, external to the vacuum portion of the device, and pole pieces extending into the vacuum portion and terminating in the region of the vanes 18, provide the requisite magnetic field within the device. The device as has been described .thus far is that which is well known in the art both in structure and operation and is intended as illustrative and not limitative.

Positioned at an angle to the wave guide 42 there is provided a'vibrating reed assembly 50 of the present invention. While this assembly 50 may be positioned at any angle with respect to the wave guide 42 especially in the range of from approximately 45 to 135, best apparent results have been achieved when this assembly is positioned as illustrated in the drawings, that is at approximately 90 from the output wave guide 42. The assembly 50 is comprised of an outer casing 52, of suitable material such as copper, which extends through the outer member 12 into the central region of the device and is terminated in close proximity to the vanes 18. The casing 52 may be vacuum sealed to the member 12 by a suitable peripheral braze 54. Disposed within the casing 52 is a reed support member 56. The support member 56 may be secured and vacuum sealed to the casing 52 by suitable means such as a peripheral braze 58. A reed 60, which will be more fully discussed later and which may best be seen with respect to FIGS. 3 and 4, is secured in a cantilever fashion to the support 56 by suitable means such as a pair of screws 62. The reed 66 is of sufficient length to extend beyond the member 14 and over at least one of the resonant cavities 20. Also supported by the member 56 is a pole piece 64 (e.g., soft iron) which is of sufficient length to extend through an aperture 56, within the reed 60, and for a distance therebeyond. A second pole piece 68, which is substantially cup shaped, is supported from the pole piece 64 by means of a spacer 70 which is of non-magnetic material. The two pole pieces 64 and 68 are dimensioned and positioned such that a portion of the pole piece 64 extends into the cup of the pole piece 68.

' Secured to the lower side of the reed 60 and positioned about the aperture 66 is a sleeve member 72, of suitable material such as non-magnetic stainless steel. Around the sleeve 72 there is formed a coil 74 of electrically conductive wire. The sleeve and coil are of such length to extend around that portion of pole piece 64 which extends through the reed 60 and into the cup portion of the pole piece 68. Suitable conductors 76, extending and vacuum sealed through the support 56 and insulated therefrom by bushings 80, are provided with terminals 78 in order that the coil 74 may be provided with a suitable source of electrical potential.

Positioned near to the free extremity of the reed 60 and secured thereto, there is provided at least one cavity tuning member 82. In the illustrated embodiment, three such cavity tuning members are shown and positioned such that they correspond respectively to three successive cavities 20. While the actual material used for the tuning members 82 is not critical, it has been found that molybdenum pins which have been clad with platinum are easily brazed to the reed 60 and also provide good resistance against decay caused by any arcing in the cavities 20.

' In operation, the two pole pieces 64 and 68, which are located within the field of the magnets 46, serve to establish a magnetic field which extends radially between the pole pieces 64 and 68. The movement of the reed 60 will in turn vary the degree of extension of the cavity tuning member 82 into the cavities 20 and, inasmuch as the members 82 are of a material to tune the cavities 20, the resonant frequency of these cavities will be varied in accordance with the degree of extension of the cavity tuning members 82 thereinto. By properly dimensioning the entire system, when the reed 60 is at its uppermost position, no part of the members 82 will extend into the cavity. When the reed is at its other extremity of oscillation, a maximum amount of the cavity tuning member 82 will be positioned within the confines of the cavity 20.

A further refinement, which is illustrated in FIG. 5, involves the placement of a second coil 75 positioned about the sleeve member 72. This coil 75 is preferably wound simultaneously with the coil 74 so as to be in a transformer relationship therewith. In this embodiment one end of each of the coils could be attached to ground and the other ends thereof each connected respectively to one of the terminals 78. As the reed motion is effected, and hence the coil 75 is passed through the magnetic field between the pole pieces 64 and 68, an electrical current is induced within this latter coil. This current may be, through suitable connecting means 79 utilized as a feedback monitor signal to amplitude modulate the voltage applied to the coil 74 and may also be utilized as an indication of reed position.

As a practical illustration of the utilization of the present device it is to be assumed that the natural resonant frequency of the cavity 20 is approximately 9,000 megacycles per second and that the resonant frequency of a cavity 20 into which has been placed the full extension of a cavity tuning member 82 is 9,030 megacycles per second. It will also be assumed, for purposes of this illustration, that the device is to be operated in a pulsed manner and at a pulse frequency of about 1200 cycles per second, If now the coil 74 is pulsed periodically at a frequency one-half that of the pulse rate of the magnetron, and these latter pulses are applied simultaneously with alternate pulses of the magnetron, it is readily apparent that during the first of successive magnetron pulses the tuning members 82 will be at their maximum disposition within the cavity while during the second of successive magnetron pulses the tuning members 82 will be entirely removed from the cavity 20. Thus, the output of the magnetron will show alternate pulses of first and second frequencies as set forth above, i.e., 9,000 megacycles per second and 9,030 megacycles per second. In order to achieve the proper physical vibration of the reed 60 it has been found desirable, although not necessary, to provide a reed the natural frequency of which is approximately the same as the frequency at which the coil 74 is pulsed.

The reed 60 itself may be best explained with respect to FIGS. 3 and 4. As is readily observable, the reed 60 is comprised of an elongated, substantially flat strip of material. In the selection of the reed material several properties had to be considered. Included in these properties are thermal conductivity, electrical properties and vibrating properties, including such things as damping, amplitude of vibration, frequency of vibration and bandwidth. While any non-magnetic or only slightly magnetic material including annealed' and unannealed bronze, copper-nickel, stainless steel, and many others will function as the reed material best results have been obtained with copper and copper base alloys. Two apertures 84 are provided near one end of the reed 60 to facilitate the mounting, by screws (See FIG. 2) to the support member 56. An important feature of the reed which was omitted from FIGS. 1 and 2 for purposes of clarity is best illustrated with respect to FIG. 3. Positioned near the free end of the reed 60,

that is the end to which are secured the cavity tuning members or pins 82, there are provided a plurality of slots 86 which extend through the thickness of the reed 60. These slots serve to break up the radio frequency currents which are set up in the reed from the oscillating currents in the anode. Since in the illustrated embodiment, there is an odd number of pins protruding into an odd number of cavities, a net current is set up on the reed. These currents if allowed to remain unchecked would cause radio frequency energy loss through heating and radiation and hence a decrease in the efficiency of the discharge and possibly generating a sufficient amount of heat to affect other parts of the assembly 50. The copper of the reed is a good material because it is a good electrical and thermal conductor. A poor electrical conductor placed in the high current cavities would dissipate a large amount of energy purely from series loss in the material. The slots 86 serve to break up what radio frequency currents are present. It has been found that the use of the structure described actually results in little or no efficiency decrease in-the operation of the discharge device and it is obvious that should the reed fail the device may still be used as a standard magnetron. Additionally, the reed may even be deactivated and the device used in its standard form.

A further feature of the reed 60 which should be pointed out is the manner in which the reed 60'may be frequency adjusted. Prior to the insertion of the assembly 50 into the resonant cavity device the assembly, either with or without the casing 52, may be placed in a weak magnetic field simulating that of the magnets 46 of the final device and the coil 74 activated in a manner similar to that as it would be in its final operation. By suitable measuring means, e.g., a stroboscopic device, the frequency of the vibration of the reed may be measured. If the frequency of the reed is not at the desired frequency, it may be varied by the simple expedience of either removing or adding material to the reed in the area 90 (FIG. 3) which is set off by dot dash lines. This area 90 is preferably immediately adjacent the area of contact with the support 56 when the reed 60 is mounted thereon. Thus, the reed may be thoroughly calibrated and adjusted prior to its insertion into the device in which it is to be operable. Preferably, the reed is designed so that its initial resonant frequency is slightly above the desired frequency so that material must be removed from the area 90. This results in a slight weakening of the reed in that area so that the free portion of the reed vibrates as a rigid member from this area.

It is thus seen that there has been described and shown a resonant cavity device and a tuning assembly for this device which will accurately and effectively vary the resonant frequency of the cavity in a predescribed manner.

While there has been shown and described what is at present considered to be the preferred embodiment of the invention, modifications thereto will readily occur to those skilled in the art. For example, while the device has thus far been described as one in which the frequency is varied in a periodic manner, it is readily evident that if, rather than pulses being applied to the coil 74, a continuous voltage were thus applied that a command type of operation could be achieved in the output frequency to provide that a variation in the output frequency between the tuned and the natural cavity frequencies could be achieved at command. It is also obvious that more than one such reed assembly could be included in a particular device.

It is not desired, therefore, that the invention be limited to this specific arrangement shown and described and it is intended to cover in the appended claims all such modifications as fall within the true spirit and scope of the invention.

We claim as our invention:

1. A magnetically operable vibrating reed assembly comprising a support, an elongated reed having one end thereof secured to said support with the other end thereof remaining free to vibrate, said elongated reed having a plurality of longitudinally extending slots therein adjacent the free end of said reed to break up any radio frequency currents induced in said reed, coil means secured to said reed, means to supply an electrical current through said coil means, and means for the establishment of a magnetic field perpendicular to the longitudinal axis of said coil whereby said reed is caused to vibrate.

2. A magnetically operable vibrating reed assembly comprising a support, an elongated reed having one end thereof secured to said support with the other end thereof remaining free to vibrate, said elongated reed having a plurality of longitudinally extending slots therein adjacent the free end of said reed to break up any radio frequency currents induced in said reed, coil means secured to said reed, means to supply an electrical current to said coil means and means inclusive of first and second magnetic pole pieces for the establishment of a magnetic field perpendicular to the longitudinal axis of said coil whereby said reed is caused to vibrate.

3. In a microwave device having a plurality of resonant cavities, an assembly including an elongated flexible member fixed at one end and free at the other end, a plurality of cavity tuning members secured to the free end of said flexible member and extendible into a plurality of said cavities, and magnetic pole means positioned between said fixed ends and said cavity tuning members for effecting a magnetic field substantially parallel to said flexible member and a coil mounted on said flexible member within said magnetic field for moving said flexible member whereby the degree of extension of said tuning members into said cavities is varied in response to current flowing through said coil.

4. A magnetron including a cathode and an anode including a plurality of circularly arranged cavitiesabout the axis of said magnetron and resonant at a first frequency, output means for removing energy from said anode, and means for varying the "resonant frequency of said cavities, said means inclusive of a vibrating cantilever member having a plurality of pin tuning members affixed to the free end thereof, successive pins insertable into successive cavities, said cantilever member having a plurality of slots therein to break up any radio frequency currents induced therein, and means for effecting the vibration of said cantilever member whereby said pins are intermittently disposed within said cavities whereby the resonant frequency of said cavities is varied.

5. A magnetron including a cathode and an anode including a plurality of circularly arranged cavities resonant at a first frequency, output means for removing energy from said anode, and means for varying the resonant frequency of said cavities, said means including a cantilever member having an odd number of pin tuning members affixed to the free end thereof, successive pins insertable into successive cavities, said cantilever member having a plurality of slots disposed therein near the free end thereof to break up any radio frequency currents induced in said cantilever member, and means for effecting the vibration of said cantilever member whereby said pins are intermittently disposed within said cavities whereby the resonant frequency of said cavities is varied.

6. A magnetron having a cavity, an assembly including an elongated flexible member fixed at one end and free at the other end, a cavity tuning member secured to the free end of said flexible member and extendable into said cavity, magnetic pole means disposed for effecting a magnetic field substantially parallel to said flexible member and a coil mounted on said flexible member between said fixed end and said free end of said flexible member and within said magnetic field for moving said flexible member whereby the degree of extension of said tuning member into said cavity is varied in response to current flowing through said coil.

7. A magnetron having a plurality of cavities, an assembly including an elongated flexible member fixed at one end and free at the other end, a cavity tuning member secured to the free end of said flexible member and extendable into one of said cavities, magnetic pole means disposed for effecting a magnetic field substantially parallel to said flexible member and a coil mounted on said flexible member between said fixed end and said free end of said flexible member and within said magnetic field for moving said flexible member whereby the degree of extension of said tuning member into one of said cavities is varied in response to current flowing through said coil.

8. A magnetron including a centrally disposed cathode, an anode including a plurality of resonant cavities circularly disposed about said cathode, output means coupled to one of said resonant cavities for removing energy from said magnetron, means adjacent said anode for varying the resonant frequency of said magnetron, said means including an elongated flexible member fixed at one end and free at the other end, a cavity tuning member secured to the free end of said flexible member and extending into at least one of said cavities, magnetic pole means positioned between said fixed end and said free end of said flexible member for providing a magnetic field substantially parallel to said flexible member and a coil mounted on said flexible member within said magnetic field for moving said flexible member whereby the degree of extension of said tuning member into said cavity is varied in response to current flowing through said coil, said cavity tuning member spaced about said circularly disposed resonant cavities from said output means in the range of 45 to 135.

9; A magnetron including a centrally disposed cathode, an anode including a plurality of resonant cavities circularly disposed about said cathode, output means positioned at a point on said circularly disposed resonant cavities for removing energy from said anode cavities, means adjacent said anode for varying the resonant frequency of said cavities, said means including an elongated flexible member fixed at one end and free on the other end, a cavity tuning member secured to the free end of said flexible member and extendable into at least one of said resonant cavities, said flexible member being positioned at a point on said circularly disposed resonant cavities approximately from said output means, magnetic pole means positioned between said fixed end and said free end of said flexible member for effecting a magentic field substantially parallel to said flexible member and a coil mounted on said flexible member within said magnetic field for moving said flexible member whereby the degree of extension of said tuning member into said cavity is varied in response to current flowing through said coil. 

1. A magnetically operable vibrating reed assembly comprising a support, an elongated reed having one end thereof secured to said support with the other end thereof remaining free to vibrate, said elongated reed having a plurality of longitudinally extending slots therein adjacent the free end of said reed to break up any radio frequency currents induced in said reed, coil means secured to said reed, means to supply an electrical current through said coil means, and means for the establishment of a magnetic field perpendicular to the longitudinal axis of said coil whereby said reed is caused to vibrate.
 2. A magnetically operable vibrating reed assembly comprising a support, an elongated reed having one end thereof secured to said support with the other end thereof remaining free to vibrate, said elongated reed having a plurality of longitudinally extending slots therein adjacent the free end of said reed to break up any radio frequency currents induced in said reed, coil means secured to said reed, means to supply an electrical current to said coil means and means inclusive of first and second magnetic pole pieces for the establishment of a magnetic field perPendicular to the longitudinal axis of said coil whereby said reed is caused to vibrate.
 3. In a microwave device having a plurality of resonant cavities, an assembly including an elongated flexible member fixed at one end and free at the other end, a plurality of cavity tuning members secured to the free end of said flexible member and extendible into a plurality of said cavities, and magnetic pole means positioned between said fixed ends and said cavity tuning members for effecting a magnetic field substantially parallel to said flexible member and a coil mounted on said flexible member within said magnetic field for moving said flexible member whereby the degree of extension of said tuning members into said cavities is varied in response to current flowing through said coil.
 4. A magnetron including a cathode and an anode including a plurality of circularly arranged cavities about the axis of said magnetron and resonant at a first frequency, output means for removing energy from said anode, and means for varying the resonant frequency of said cavities, said means inclusive of a vibrating cantilever member having a plurality of pin tuning members affixed to the free end thereof, successive pins insertable into successive cavities, said cantilever member having a plurality of slots therein to break up any radio frequency currents induced therein, and means for effecting the vibration of said cantilever member whereby said pins are intermittently disposed within said cavities whereby the resonant frequency of said cavities is varied.
 5. A magnetron including a cathode and an anode including a plurality of circularly arranged cavities resonant at a first frequency, output means for removing energy from said anode, and means for varying the resonant frequency of said cavities, said means including a cantilever member having an odd number of pin tuning members affixed to the free end thereof, successive pins insertable into successive cavities, said cantilever member having a plurality of slots disposed therein near the free end thereof to break up any radio frequency currents induced in said cantilever member, and means for effecting the vibration of said cantilever member whereby said pins are intermittently disposed within said cavities whereby the resonant frequency of said cavities is varied.
 6. A magnetron having a cavity, an assembly including an elongated flexible member fixed at one end and free at the other end, a cavity tuning member secured to the free end of said flexible member and extendable into said cavity, magnetic pole means disposed for effecting a magnetic field substantially parallel to said flexible member and a coil mounted on said flexible member between said fixed end and said free end of said flexible member and within said magnetic field for moving said flexible member whereby the degree of extension of said tuning member into said cavity is varied in response to current flowing through said coil.
 7. A magnetron having a plurality of cavities, an assembly including an elongated flexible member fixed at one end and free at the other end, a cavity tuning member secured to the free end of said flexible member and extendable into one of said cavities, magnetic pole means disposed for effecting a magnetic field substantially parallel to said flexible member and a coil mounted on said flexible member between said fixed end and said free end of said flexible member and within said magnetic field for moving said flexible member whereby the degree of extension of said tuning member into one of said cavities is varied in response to current flowing through said coil.
 8. A magnetron including a centrally disposed cathode, an anode including a plurality of resonant cavities circularly disposed about said cathode, output means coupled to one of said resonant cavities for removing energy from said magnetron, means adjacent said anode for varying the resonant frequency of said magnetron, said means including an elongated flexible member fixed at one end and free at thE other end, a cavity tuning member secured to the free end of said flexible member and extending into at least one of said cavities, magnetic pole means positioned between said fixed end and said free end of said flexible member for providing a magnetic field substantially parallel to said flexible member and a coil mounted on said flexible member within said magnetic field for moving said flexible member whereby the degree of extension of said tuning member into said cavity is varied in response to current flowing through said coil, said cavity tuning member spaced about said circularly disposed resonant cavities from said output means in the range of 45* to 135*.
 9. A magnetron including a centrally disposed cathode, an anode including a plurality of resonant cavities circularly disposed about said cathode, output means positioned at a point on said circularly disposed resonant cavities for removing energy from said anode cavities, means adjacent said anode for varying the resonant frequency of said cavities, said means including an elongated flexible member fixed at one end and free on the other end, a cavity tuning member secured to the free end of said flexible member and extendable into at least one of said resonant cavities, said flexible member being positioned at a point on said circularly disposed resonant cavities approximately 90* from said output means, magnetic pole means positioned between said fixed end and said free end of said flexible member for effecting a magentic field substantially parallel to said flexible member and a coil mounted on said flexible member within said magnetic field for moving said flexible member whereby the degree of extension of said tuning member into said cavity is varied in response to current flowing through said coil. 